In mobile broadband cellular communication systems, there are several physical layer techniques that require a transmitter to be provided with knowledge of the channel response between the transmitter and a receiver. Transmission techniques that make use of the channel response between the transmitter and receiver are called closed-loop transmission techniques. One example of closed-loop transmission is the use of transmit precoding at the transmitter. An antenna array employing transmit precoding comprises of an array of multiple transmit antennas where the signals fed to each antenna are weighted in such a way as to control the characteristics of the transmitted signal energy according to some pre-defined optimization strategy.
Generally, the transmitted antenna signals are weighted based on knowledge of the space-frequency channel response between each transmit antenna and each receive antenna and attempts to optimize the characteristics of the received signal processed by the receiving device.
In general, there are different techniques for providing a transmitter with knowledge of the channel between each transmit antenna and each receive antenna. The methods described henceforth are applicable to any multiple-antenna equipped wireless transmitter and a receiver. However, for the sake of clarity, this discussion is focused at the downlink of a cellular system where the base station (BS) is the transmitter and a mobile station or subscriber station (SS) is the receiver.
One technique is based on feedback messages from the SS, where the SS measures the channel response between the BS antennas and the SS antennas and transmits a feedback message back to the BS containing enough information that enables the BS to perform closed loop transmit preceding. This technique relies on digital signaling that includes codebook based quantization at the SS and encoding the precoding matrix index as a feedback message.
Another technique is based on the reciprocity of the RF channel response. An RF propagation channel may be treated as reciprocal (by virtue of antenna calibration), which means the downlink RF channel matrix (where the matrix refers to the channel gains between each transmit and receive antenna) at a given time-frequency point is simply the matrix transpose of the uplink RF channel matrix at the same time-frequency point. In a TDD system, a downlink channel response can sometimes be derived from an uplink data transmission. Along the same lines, in an FDD system some direction-of-arrival (DOA) based methods may be used to derive spatial properties of a downlink channel from uplink transmission.
Another technique is to feedback analog channel information measured by the SS via an uplink channel. This analog channel information may include spatial covariance matrix coefficients, channel coefficients, precoding matrix coefficients.
Accordingly, what is needed is a technique to feedforward precoding information from a BS such that the SS can reconstruct the downlink channel response, properly decode data, and perform closed loop reception. It would also be of benefit to provide a technique that does not rely on digital signaling. It would be of further benefit to provide a technique that does not rely on sounding based on beamformed pilot signals.
Skilled artisans will appreciate that common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted or described in order to facilitate a less obstructed view of these various embodiments of the present invention.